1. Field of the Invention
The present invention relates to sensors that utilize a sensing element and electronic circuits in performing a sensing function to measure the value of a physical parameter, such as linear or rotary position, and that provide an electrical output indicative of the value of the measured parameter. Further, the present invention relates to sensors operating in an environment in which it is desirable to avoid the use of ferromagnetic materials, to minimize the physical size of the sensor, or in which a high temperature or other harsh environmental condition, such as ionizing radiation, may exist. Still further, the present invention also relates to high reliability measurements in which multiple sensors may be deployed in order to provide redundant measurements, or to sensor installations in which it is desired to connect a minimum number of wires to the sensor.
2 . Description of the Prior Art
Prior Art sensors, such as Linear Variable Differential Transformers (LVDTs), and magnetostrictive position sensors, typically utilize a sensing element and an electronics module. The electronics module powers the sensing element, conditions the signal provided from the sensing element, and provides a desired output signal. The signal conditioning part of the electronics module is designed specifically to provide the type of power and to receive the type of signal provided by the sensing element. The included analog and/or digital circuits for powering and signal conditioning are generally somewhat complex, and the required specialized components are difficult or impossible to find on the market with maximum temperature capability of more than 125° C.
Prior Art sensors can sometimes be physically partitioned into a sensing part, and a signal conditioning part, so that the sensing part can be exposed to a higher temperature (or sometimes, to radiation), and the signal conditioning part can remain at a lower temperature (or, at a lower radiation level). For example, an LVDT can be separated from the signal conditioner, and connected therewith by three to six power and signal wires, allowing the LVDT to be located in a higher temperature area. But the connecting wires carry sinusoidal and quadrature sinusoidal waveforms at kilohertz (kHz) frequencies, and sometimes at relatively low voltage levels, while the amplitude of the voltage, as well as the phase, represents the signal. So, the connections must be made using a shielded cable, and must be used with a signal conditioner that is designed for that type of LVDT, and that is calibrated for exactly that particular LVDT.
With a magnetostrictive linear position sensor, as another Prior Art example, the sensing head can be separated from the conditioning electronics by a very short cable having wires for the interrogation pulse and wires for the received signal pulse. But the interrogation wires must carry current in the range of more than ten volts and more than one ampere, while the signal wires carry a signal in the millivolt and micro amp range. This places limitations on the length (a few inches) and type of cable (individually shielded pairs), and requires a specialized electronics module that is designed for the type of sensing element and that is calibrated for exactly that particular magnetostrictive sensor. Many Prior Art sensors include magnetic materials, such as iron and nickel, and permanent magnets, which are sometimes not compatible with requirements of a specific application. An LVDT uses a core made from a ferromagnetic material (usually a nickel-iron alloy), and magnetostrictive position sensors utilize a position magnet which is a permanent magnet, often a rare-earth magnet.
In Prior Art high reliability applications, multiple identical sensors have often been deployed to make one measurement. With two sensors (a dual-redundant system), as long as the two sensors agree, then the data are expected to be accurate. If the two sensors are reporting different readings, then the data from both sensors are suspect, and should not be relied upon. With three sensors (a triple-redundant system), as long as at least two of the sensors agree, then the system can continue to operate with this value until a convenient time for service and replacement of the one sensor that disagrees. Dual and triple-redundant sensor systems have been deployed in the Prior Art by installing two or three separate sensors, and then mechanically coupling each of them to the same movable object, so that they each measure the position of the movable object. This mechanical coupling introduces errors due to differences in alignment, free-play, and other imperfect attributes of the mechanical couplings.
In U.S. Pat. No. 4,637,265, a non-contact sensor apparatus uses the combination of a stationary coil and a movable coil, connected into stationary and movable tank circuits, which are inductively coupled to produce a double resonance curve in the stationary tank circuit. But, having a movable coil and electronics assembly, this arrangement is not suitable for separation of the sensing element from the electronics module.
In U.S. Pat. No. 7,216,054, a non-contact position sensor is taught that can be fabricated at low cost and with high reliability. According to that invention, multiple sensors would still be deployed in the case where redundant measurements are needed, but instead, the present invention can be applied to such a sensor, and thereby all of the advantages of the present invention would be added.